Abstract
Sm2(Co,Cu,Fe,Zr)17 are the best high-temperature permanent magnets because of their high Curie temperature (800°C–850°C). However, irreversible and unacceptable coercivity losses retard their use in applications at temperatures over 550°C. The coercivity loss has been correlated with poor oxidation resistance at high temperatures. The current research progress on the effect of oxidation and its prevention, for 2:17-type magnets, is reviewed. Oxidation in air at 500°C–700°C causes the magnets to form three regions: (1) an external oxide scale mainly consisting of (CoxFe1-x)3O4, (2) a thicker internal oxidation zone where the typical cellular precipitation (2:17R cell and 1:5H cell boundary) structure has been completely collapsed due to the Sm oxidation into Sm2O3, and (3) an oxidation-free zone where the cellular precipitates remain unchanged in lattice structure. No unacceptable coercivity loss is seen in the oxidation-free zone. Its thickness can be impressively increased within the magnets at high temperature, when they are covered with surface diffusion barriers for oxygen from the atmosphere, such as thin films of Cr2O3, Al2O3, and the metals with the ability to thermally grow these oxides.
About the authors
Xiao Peng obtained his PhD degree in Corrosion and Protection at the Institute of Corrosion and Protection of Metals, Chinese Academy of Sciences. He was a post-doctorate researcher at the Materials Department of the University of California at Santa Barbara from 1998 to 1999 and a visiting scientist at the Chemistry Department of Western Kentucky University from 1999 to 2001. He has been a Professor at Institute of Metal Research, Chinese Academy of Sciences, since 2001. He has 25 years’ experience in high-temperature corrosion and protection of metallic materials. He has published over 100 peer-reviewed journal articles and over 10 invited presentations for international conferences.
Hongbo Zhao studied Metallurgical Science and Engineering at the Central South University of China between 2006 and 2010. In 2010, he was recommended to work on his PhD degree in corrosion and protection of metals without exams under Prof. Xiao Peng’s guidance at Institute of Metal Research, Chinese Academy of Sciences. He is currently doing his PhD research mainly on high-temperature oxidation and protection of samarium-cobalt permanent magnets.
Xiaolan Wang received her Master’s degree in Materials Science from the Institute of Metal Research, Chinese Academy of Sciences, in 2005. She is currently an engineer at the Institute of Metal Research and pursuing a PhD degree. Her current research focuses on the understanding of high-temperature oxidation of materials in terms of TEM studies.
Zhaohui Guo received his MS degree in Condensed Matter Physics at the Institute of Physics, Chinese Academy of Sciences, in 1996. Then he joined the Division of Functional Materials at the Central Iron and Steel Research Institute, where he obtained his PhD in Materials Science and engineering in 2006. He is currently a Professor at the institute, responsible for R&D on high-temperature rare earth permanent magnets and nanocomposite permanent magnets. Dr. Guo has more than 20 years’ experience and has published over 40 technical articles on rare earth magnets. He has obtained several National Scientific and Technological Progress Awards of China.
Wei Li studied Magnetism at Shandong University between 1978 and 1982. Since 1984, he has been a Professor at the Central Iron and Steel Research Institute, doing research on the microstructure, magnetic, mechanical, and corrosion properties of rare earth permanent magnetic materials. He has over 30 years’ experience and has published over 150 articles. He is an elected member of the Asia Pacific Academy of Materials and a director of the Chinese Society of Rare Earths. He won one First-Class Prize and two Second-Class Prizes for National Scientific and Technological Progress Awards of China.
Fuhui Wang received his PhD in Corrosion and Protection at Institute of Corrosion and Protection of Metals, Chinese Academy of Sciences, in 1992. He was a deputy director of Institute of Corrosion and Protection of Metals and the director of State Key Lab for Corrosion and Protection. He worked at the Technical University of Munich, Germany, for 1 year and at University of New South Wales, Australia, for 6 months. He is a Professor at the Institute of Metal Research and is the president of the Chinese Society for Corrosion and Protection. His main research interests are high-temperature corrosion and protection, particularly on coating development and synergistic effect of NaCl and water vapor on metal corrosion.
Acknowledgments
The work is supported by National Basic Research Program (No. 2010CB934604) of China, Ministry of Science and Technology.
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